Depositional
Environments, Cyclostratigraphy, and Paleoclimatic Signals of Lower to Middle Jurassic Fluvial-Lacustrine Deposits, Northern Bogda Mountains, Southeastern Junggar Basin, Northwestern
China
Yang, Wan1, Qiang Guan2, Jihong Li3,
Xiaohui Zhang3, Jun Han2, Wei Guan3, Fan Song3, Wenqiang Yang3 (1) Wichita State University,
Wichita, KS (2) East Junggar Division, Xinjiang Petroleum Bureau, PetroChina,
Fukang, China (3) Department of Geology, Northwestern
University, Xian, China
A 2112-m Lower-Middle Jurassic section
was measured at a cm-dm scale in northwestern Bogda Mountains, NW China. It contains
fluvial-lacustrine rocks of upper Lower-Jurassic
upper Badaowan and Sangonghe,
and Middle-Jurassic Xishanyao and Toutunhe
formations. Five types of cycles were identified. 357 lakeplain-littoral
cycles contain upward carbonaceous shale/coal, tabular sandstone, interbedded siltstone and shale, and paleosols.
They are 1-3 m thick, interpreted as lake
transgression/expansion-regression/contraction cycles. 306 thin deltaic cycles
consists of upward thin carbonaceous shale/coal, tabular sandstone, an
upward-coarsening succession of shale, siltstone, and sandstone, and paleosols. They are1-5 m thick, as lake
expansion-embayment fill cycles. These two cycle types suggest
high-frequency lake-level oscillation, indicating high-frequency precipitation
seasonality. 83 thick deltaic cycles contain pronounced
upward-coarsening-and-thickening successions of shale, siltstone, sandstone,
and conglomerate. They are 20-30 m thick, stacked into prominent sandstone-rich
intervals. Five braided-stream cycles of conglomerate and sandstone stack at
basal Sangonghe Formation. 16 meandering-stream
cycles of conglomerate, sandstone, shale, and paleosols
scatter throughout the section. Lakeplain-littoral
and thin deltaic cycles thin upsection. They
alternate with braided-stream and thick deltaic cycles, forming 15
intermediate-order cycles suggesting episodes of increased coarse clastic influx, subsidence, source
uplift, and/or precipitation that punctuated prolonged lakeplain-littoral
sedimentation. Paleosols change from thin
well-developed Histosols and rare immature Calcisols in Badaowan-Sangonghe
cycles, thick coal-rich Histosols and minor Spodosols in Xishanyao cycles, to
variegated Argillisols and poorly-developed Histosols in Toutunhe cycles. A
long-term subhumid-humid, humid, to subhumid-semi-arid climate change is inferred, probably
caused by waning Tethys-Laurasia megamonsoon
due to decreased continentality during Pangea breakup.